Polyurethane foams are widely known and used in automotive, housing and other industries. Foam is generally referred to as rigid, microcellular, or flexible. The commercial production of polyurethanes via isocyanate polyaddition reactions requires the use of catalysts. Tertiary amines are widely accepted in the industry as versatile polyurethane catalysts. They are generally stable in the presence of standard polyurethane formulation components and can impact both the blowing (water-isocyanate) and gelling (polyol-isocyanate) reactions. Unfortunately, a number of commonly used tertiary amine catalysts are relatively volatile due to low molecular weight and are malodorous and offensive. Release of tertiary amine during foam processing may present significant safety and toxicity problems, and release of residual amines from consumer products is generally undesirable. Therefore, identifying alternatives to standard tertiary amine catalysts which are not malodorous, yet exhibit the same type of activity in isocyanate polyaddition reactions would be desirable.
One strategy for the reduction of odor associated with tertiary amine catalysts is the identification of less volatile structures. The prior art teaches numerous techniques for reducing volatility, particularly increasing molecular weight or improving hydrogen bonding capability. Amine catalysts which contain primary and/or secondary hydroxyl functionality typically have limited volatility and low odor when compared to related structures which lack this functionality. Furthermore, catalysts which contain hydroxyl functionality chemically bond into the urethane during the reaction and are not released from the finished product. Catalyst structures which embody this concept are typically of low to moderate activity and are designed to promote the blowing reaction.
U.S. Pat. No. 4,026,840 discloses the use of hydroxy-functionalized tertiary amines as polyurethane catalysts, particularly useful for the production of polyisocyanurate.
G.B. 1,153,308 discloses polyurethane foam compositions containing N-hydroxyalkyl substituted imidazoles used as gelling catalysts.
BE 671,012 discloses the use of 1,2-disubstituted imidazoles, preferably 1,2-dimethylimidazole, 1-(2-hydroxyethyl)-2-methyl-imidazole and 1-(2-hydroxypropyl)-2-methylimidazole in polyester polyol based rigid, semi-rigid, or flexible foams based on toluene diisocyanate (TDI) and 4,4'-diphenylmethane diisocyanate (MDI).
U.S. Pat. No. 4,857,560 discloses reduced odor emission as an additional advantage of catalysts formed from the reaction of tertiary amines and an acid mixture of 1) boric acid and 2) a carboxylic acid.
U.S. Pat. No. 5,238,894 discloses hydroxyl-containing amine-boron adducts as reduced odor compositions for the production of polyurethanes. The catalysts consist essentially of a hydroxyl-containing tertiary amine urethane catalyst and a boron compound of the formula R.sub.n B(OH).sub.3-n where n=0 or 1. However, boric acid derivatives were not used in combination with hydroxy-functional imidazole compounds. In addition, the use of boron compounds as a way to alter the blow/gel selectivity of the catalyst was not discussed.
CA 99(6):39229a notes that boric acid, when used as a filler at 10-40%, decelerates foaming in polyurethanes based on Laprol 805 and Lapramol 294 polyether polyols, polyisocyanate, Freon 113 blowing agent and water. "Int. Prog. Urethanes 1980", 2, 153-73 describes the use of boric acid as a blowing agent, its behavior being almost equal to that of water. Neither reference describes the effect of boric acid at use levels typified by a catalyst, nor do they indicate the effect of boric acid on polyurethane catalysis in the presence of a tertiary amine.
U.S. Pat. No. 4,611,013 describes the use of quarternary ammonium borates to effect the concurrent trimerization/carbodiimidization of polyisocyanates. The borates are prepared from boric acid, alcohols and quarternary ammonium hydroxides and as such are not derived form tertiary amines. Other examples are given by U.S. Pat. No. 4,530,938 and 4,425,444.
U.S. Pat. No. 3,193,515; U.S. Pat. No. 3,127,404 and FR 2,301,554 disclose the use of boric acid in the preparation of an ammonium salt polyurethane catalyst from triethylenediamine and a glycol borate acid.
"J. Org. Chem." 1972, 37 (14), 2232 discloses that monodentate nitrogen nucleophiles do not react significantly with boric acid in aqueous solution.